This application is directed to methods of applying additives to a forming web of paper fiber at a paper machine wet end.
Direct application of highly fibrillated cellulose or other additives to a sheet surface of a forming paper web can greatly enhance the surface properties and provide a superior surface for coating applications. Highly fibrillated cellulose or other additives can be applied with a vertical curtain falling by gravity, but the curtain has a much lower velocity in the machine direction than the forming paper web on which it lands, which causes significant extension of the additive layer and causes stresses and disruptions both in this layer and in the top surface of the paper web. Pressurizing the applicator to increase the velocity of the vertical curtain causes the curtain to partially penetrate and disrupt the forming paper web as it lands. Known alternative technology to apply liquid horizontally on a paper machine wet end is a secondary headbox, used for making multi-ply paper grades. However, this technology is not suitable for adding additives such as highly fibrillated cellulose for two reasons.
The very small-scale fibers or particles such as fibrillated cellulose must be kept in high speed, micro-turbulent flow during the entire delivery period to prevent settling out and to maintain a good fluid dispersion. For this purpose, a conventional headbox slice zone, with a large converging channel ending with an essentially vertical top slice lip, is not suitable. See, for example, U.S. Pat. No. 7,279,043. Other patents disclosing conventional headboxes include U.S. Pat. Nos. 4,141,789 or 4,285,787.
The minimum flow rates possible with secondary headbox technology are too high, meaning that significant extra drainage capacity would be needed on the paper machine wet end to drain the additional water load, or the machine speed would have to be significantly reduced.
U.S. Pat. Nos. 5,792,317 and 5,985,030 disclose an applicator for applying a suspension of uncooked starch or other additives in the form of a curtain to a forming web on a paper machine. In these patents an additive is forced out of a narrow slot in the applicator and falls downward onto the forming paper web.